1. Field of the Invention
The present invention relates to a lead wire used for a sensor or detector such as an oxygen concentration sensor or detector.
2. Description of the Prior Art
As the lead wire for an oxygen concentration sensor, a plurality of core wires bundled together have been used in the past. In general, copper is used as the core wires, whereas stainless steel wires are also used in some rare cases. It has also been proposed to use both copper wire and stainless steel wire as lead wires (Japanese Utility Model Publication 4-52887 and Japanese Patent Publication Laid-Open 56-35656).
However, there are the following disadvantages in the conventional type lead wires used for a sensor:
First of all, when copper wires are used for all of the core wires, tensile strength of the lead wire is weak. Also, fatigue caused by bending is high, and elasticity is low. Further, in case the above lead wire is used for a sensor in an automobile, the lead wire may be damaged by pebbles and stones bounced by wheels when the car is driven. On the other hand, to improve tensile strength and elasticity of the lead wire, it has been proposed to reduce diameter of core wire. However, if the wire diameter is reduced too much, the lead wire is more frequently susceptible to damage from external forces and the cost is also extensively increased.
In case stainless steel wire is used for all of the core wires, tensile strength and elasticity of the lead wire are satisfactory, while electrical resistance of the lead wire becomes extremely high. For this reason, it is difficult to use this type of wire as the lead wire for a low-resistance heater. Also, it is difficult to freely bend and arrange the lead wire because its flexibility is low.
In the former of the above Japanese Publications, a lead wire is disclosed, which uses copper wires as core wires and stainless steel wires arranged around the core wires. In this lead wire, the ratio of stainless steel is as high as about 81%. As a result, electrical resistance is high in this type of lead wire, and this adversely affects sensor output or performance of the low-resistance heater. Accordingly, this type of lead wire is inadequate as the lead wire for these purposes.
The latter of the above Japanese Publications discloses a lead wire, which comprises a plurality of bundles of copper wires and stainless steel wires stranded together, but the ratio of stainless steel components is as low as about 22%. As a result, tensile strength is weak although electrical resistance is low.
In addition, sensors are increasingly mounted at the site with more complicated surrounding conditions, and it is often difficult to mount a sensor unless the lead wires derived from the sensor can be easily bent.
For this reason, there are increasing demands to provide lead wires that are low in electrical resistance and high in elasticity, flexibility, and tensile strength.